CBA #1 Review 2013-2014
Graphing Motion 1-D Kinematics Projectile Motion Circular Motion Gravity
Graphing Motion
Distance vs. Time
Velocity vs. time
Acceleration vs. time
Average velocity is the slope of the x vs. t graph.
Compare the velocities for the three graphs.
The graph tells you
1. The direction of motion.
2. The relative speed.
Vavg = Dx / Dt
Acceleration a = Dv / Dt
The acceleration of an object tells you how much the velocity changes every second.
The units of acceleration are m/s2.
The acceleration is the slope of a velocity vs. time graph.
a = Dv / Dt = rise / run = 3/5 m/s2.
+ slope = speeding up-slope = slowing downzero slope = constant speed
Summary
Displacement Dx = xf - xi
Average Velocity Vavg = Dx / Dt
Acceleration a = Dv / Dt
Average velocity is the slope of the x vs. t graph.
Acceleration is the slope of the v vs. t graph.
The graph tells you1. The direction of motion.2. The relative speed
The acceleration of an object tells you how much the velocity changes every second.
Constant Acceleration GraphsA cart released from rest on an angled ramp.
An object dropped from rest.
7
Positio
n
Velocity
Acceleratio
n
Time Time Time
1-D Kinematics
ExampleA car starts from rest and accelerates at 4 m/s2 for 3 seconds.
1. How fast is it moving after 3 seconds?
2. How far does it travel in 3 seconds?
1. 4 = (vf – 0) / 3 , vf = 12 m/s
2. Dd = 0(3) + .5(4)(32) = 18m
ExampleA car starts from rest and obtains a velocity of 10m/s after traveling 15m. What is its acceleration?
a = (102 – 0) / ( 30) = 3.33 m/s2
Fired Horizontally
d = ½ gt2 x = vt
Example
A ball moving at 5 m/s rolls off of a table 1m tall and hits the ground.
1. How long was it in the air?
2. What horizontal distance did it travel?
1. d = ½gt2 1 = .5(9.8)t2 t = = .45s
2. x = vt = 5(.45) = 2.26m
Example
A ball rolls off of a table and hits the ground 1.5m away after falling for .5 seconds.What was its initial velocity?
x = vt , so v =x/t = 1.5m/.5s = 3. 00 m/s
Circular Motion
Example: A car rounds the circular curve (r = 50m) in 10 seconds.
1. What is the velocity?2. What is the centripetal acceleration while in the curve?
1. V = d/t = (pr/t ) = (3.14)(50)/10 = 15.7 m/s
2. a = v2/r = (15.7)2/50 = 4.93 m/s2
Step 1: Identify all of the forces acting on the object Step 2 : Draw a free body Diagram Step 3: Break every force into x and y components. Step 4: Apply the second law: SFx = max SFy = may
This usually gives 2 equations and 2 unknowns. Step 5: If needed, apply the kinematic equations. xf = xi +vit +1/2at2 vf = vi + at
Applying Newton’s Laws of Motion
Problems With Acceleration
A box (m = 20kg) is pushed to the right with a force of 50N. A frictional force of 20N acts to the left. What is the acceleration of the box?
P = ( 50, 0 )
W = ( 0, - 196 )
N= ( 0, N )
f = ( -20,0)
SFx = max , 50 – 20 = 20 ax , ax = 1.50 m/s2
EXAMPLE
Find the net force down the plane.
max = mgsinq – f = 40sin(30) – 10 = 20 – 10 = 10N
Universal Gravity
F = m1m2G/r2
Newton’s Law of Gravity : Every two objects attract each otherwith a gravitational force given by:
m1 = mass of the first object in kg m2 = mass of the second object in kg r = distance between the two masses in meters G = 6.67 x 10-11
Example
Find the force between these two masses.
F = m1m2G/r2 = (10)(10)(6.67 x10-11)/22 =
1.67 x 10-9 Newtons